The move comes amid fears that if just one of the countries does not give a clear ‘yes’ to accession, support among other nations could plummet and the already fragile enlargement schedule could be delayed beyond 2004.

Support for joining the EU is currently as low as 35% in Estonia and 42% in Latvia and the Czech Republic, according to Eurobarometer polls taken at the end of October.

The Visegrad nations – Hungary, Slovakia, Czech Republic and Poland – have been holding talks for several months as to who will hold their referendum first.

Hungarian Prime Minister Peter Medgyessy had originally proposed 15 March as decision day but postponed the vote until 12 April.

This followed demands by opposition parties led by former leader Viktor Orban’s conservative centre-right Fidesz for more time to study the constitutional amendments that Hungary, like all candidate countries, must undertake in order to transfer part of its legislative authority to the EU.

Several other candidate countries are expected to follow suit with their polls after the Accession Treaty signing ceremony in April, as the rush to ensure that enlargement goes according to plan intensifies.

Miriam Chlupikova, a spokesperson from the Slovakian mission to the EU, predicted: “There will be a cascade of referendums.

“It’s not certain whether there will be much space in between them – it could be one or two days, or one or two months.”

Slovakia does not have a concrete date yet but is expected to hold its referendum in the first half of June.

The three Baltic States – Estonia, Latvia and Lithuania – are considering whether to hold their referendums consecutively or simultaneously – an idea particularly favoured by Lithuania.

They are also holding talks with the Visegrad countries to “elaborate exact dates within this broader group”, according to a Lithuanian spokesman.

Poland, however, remains a big worry for EU officials as it continues its attempts to wring last-minute concessions in a bid to strengthen its large but impoverished farm sector.

Although opinion polls indicate that 66% of Poles support EU membership, analysts warn that an alliance of farmers and nationalists could mount a determined anti-EU challenge if their eventual entry terms are perceived as unfair.

Poland is set to hold its referendum in June, after Slovakia and Hungary. If voter turnout in Poland is less than 50% – and that has always been the case in referenda in the post-Communist era – the result would be non-binding.

In this event, a two-thirds majority in both houses of parliament could approve entry - not a foregone conclusion as the government would need the support of its coalition partner, the Peasant’s Party, which is unhappy with the proposed EU entry terms. Ironically, public support for membership in candidate countries is much higher in Bulgaria and Romania – which are unlikely to be invited to join the Union before 2007-8.

Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia will be formally invited to join the EU at the Copenhagen summit next week.